(1) Field of the Invention
This invention relates to methods used to fabricate semiconductor devices, and more specifically to a process for creating tungsten filled contact, or via holes.
(2) Description of Prior Art
The objectives of the semiconductor industry are to continually improve the performance of silicon devices, while maintaining or decreasing the cost of these devices. The ability to manufacture silicon chips, exhibiting sub-micron features, have in part, allowed the performance and cost objectives to be realized. For example silicon devices, fabricated with specific sub-micron features, result in larger decreases in performance degrading resistances and capacitances, then counterparts fabricated using less aggressive designs. In addition the ability to produce smaller silicon chips, due to the use of smaller features, allows more chips to be produced from a specific size starting silicon substrate. Since the processing cost for the silicon substrate remains the same, the cost of an individual silicon chip is reduced. The use of sub-micron features, or the trend to micro-miniaturization, has been achieved as a result of advances or development of several semiconductor fabrication disciplines, such as photolithography, and dry etching. The use of more sophisticated exposure cameras, as well as the development of more sensitive photoresist materials, have allowed sub-micron images to be routinely produced in photoresist layers. In addition the development of selective, anisotropic, dry etching processes, have allowed these sub-micron images in photoresist layers, to be successfully transferred to underlying materials, used for the fabrication of advanced silicon devices.
However with the trend to micro-miniaturization, difficulties are encountered with specific semiconductor fabrication sequences, that had not been present for counterparts fabricated using larger device features. For example via or contact holes, used to interconnect two levels of metallization, or used to connect an overlying metallization to an underlying silicon device region, become more difficult to fill with a conductive material, such as aluminum, using metal deposition processes, such as evaporation or sputtering. Chemically vapor deposited processes supply superior fill characteristics, however it is difficult to chemically vapor deposit aluminum. In addition the use of aluminum, to fill via or contact holes, is limited due to the combination of increased current carrying demands, arising as a result of micro-miniaturization, and the poor current carrying characteristics of aluminum. Therefore the industry has preferred to use tungsten as a fill material for small diameter contact holes, taking advantage of the improved conformality properties of the chemically vapor deposited tungsten, and the enhanced current carrying characteristics of tungsten.
One specific shortcoming of the use of chemically vapor deposited, metal fills, is the seam or dimple in the fill. The metal fill proceeds by the mechanism of the depositing metal film, coating the sides of the contact hole. At a specific point, where the coated sides converge, a seam or dimple can evolve. Subsequent processing, such as dry etching used to remove unwanted metal from areas outside the contact hole, can aggravate the seam and produce a metal void, in the center of the contact or via hole. The void now offers a severe topology for subsequent interconnect metallizations, which can sometimes result in yield or reliability problems. Therefore the semiconductor industry has offered many solutions for the seam or dimple phenomena, encountered when using chemically vapor deposited metal films, specifically tungsten, to fill small diameter contact holes. One such solution, used to alleviate the dimple or seam phenomena for tungsten fills, has been offered by Ito, in U.S. Pat. No. 5,422,310. Ito suggests using a two stage dry etching process to remove unwanted tungsten, from areas outside the tungsten filled contact hole. The second stage of the dry etching procedure, performed at different conditions then the previous etching stage, remove tungsten residuals without significant attack of the tungsten in the contact hole, thus not aggravating the seam. The solution offered in this invention concentrates on a specific set of tungsten deposition conditions that allow for improved tungsten conformality, in turn allowing the fill to be successfully completed with a minimum of deposited metal. This results in a shorter etch back sequence, thus less exposure of tungsten seam to the etch back process.